1. Field of the Invention
This invention relates to a film consisting of an ultra-black film formed on a base surface and a method of forming the same. The ultra-black film is formed by etching a nickel-phosphorus alloy film deposited by an electroless plating process on the base surface. It has low spectral reflectance, has small wavelength dependence of the reflectance, then is effectively utilized as optical absorber.
2. Description of the Related Art
As the ultra-black film which is formed on a base surface, coating films using black paints, black oxide films, black metal compound films, black-chromate-treated films obtained by metal plating, black chromium films, black nickel films and films obtained by impregnating porous films formed by an anodic oxidization process with black dyes are known. These ultra-black films generally have spectral reflectance of 3 to 10 %. This reflectance is unsatisfactory if the films are to be used as optical absorber for optical power measurement apparatuses or the like. As a further variety of the ultra-black film formed on a base surface, a gold black film consisting of ultramicroparticles of gold is known. This film has a spectral reflectance of approximately 0.5 %, which is lower than those of black paints noted above. Hence, bases with this film are utilized as optical absorber for optical power measurement apparatuses or the like. The film, however, has low mechanical strength, and if it is used under conditions subjected to mechanical vibrations or rubbing, it readily drops out from the base. In addition, under high humidity conditions, it absorbs moisture which increases the reflectance. Once this occurs, the initial reflectance can not be recovered when it is dried again. For the above reasons, the environments and conditions of its use are extremely limited. U.S. Pat. No. 4,233,107 and 4,361,630 pertaining to inventions by C. E. Johnson Sr. disclose a method of obtaining a ultra-black film by etching a nickel-phosphorus alloy plating film with an aqueous nitric acid solution. This ultra-black film has spectral reflectance ranging from 0.5 to 1.0 % and has high mechanical strength. However, the optical reflectance varies greatly with wavelength change. Therefore, this film still poses problems in its use as optical absorber for measuring optical power with high accuracy in a wavelength range of 380 to 1,800 nm.
U.S. Pat. No. 4,511,614 per taining to an invention by R. L. Greeson et al discloses an improvement over the afore-noted two United States patents. The disclosed film consists of two nickel-phosphorus alloy layers having different phosphorus contents. This film, however, requires an increased number of steps of manufacture because of two-layer structure.
Further, the spectral reflectance of the film exceeds 0.5 %.